In the transmission of digitally encoded data, it is usually required to identify the beginning or end of an event, or to synchronize the receiving equipment with the transmitter. For example, in the transmission of picture data (e.g., facsimile,) it is necessary to identify the start of message, end of message, beginning of line and in other applications, one or more synchronizing signals are periodically injected during line scans to minimize loss of data. These signals must have characteristics such that they will be easily recognized by the receiver, but not mistaken for intelligence data.
One such family of coded words is classed as PN (pseudo-noise) codes whose bit length is 2.sup.n -1, where n is an integer. The choice of n is dependent on the "noisiness" of the transmission channel, i.e., the probability that noise will cause errors in the received data. The larger the value of n, the greater will be the probability of being able to differentiate between data and the PN code. In many applications, n is chosen as 5, i.e., the number of binary bits in the PN code is 31. For any n there are a maximum number of unique PN codes that can be generated. For the case of n= 5, there are six possible codes, any or all of which may be used for a given application. The number of possible codes increases with increasing n.
A further requirement for optimum performance is that the PN word be recognized as such even if it is degraded by noise, i.e., one or more bits may be in error.